Worldwide Internet access now includes more than 1.2 billion users. In concert with this growth, wireless broadband services have begun to proliferate in cities and rural communities where access was previously unavailable. Businesses around the world are also beginning to offer high-speed Internet access in restaurants, hotels, airports and other public gathering places.
Recognizing the trajectory of the growth of wireless broadband services, regulatory bodies around the world have opened new spectrum in the 5 Giga Hertz (GHz) frequency band to stimulate the availability of broadband services, increase competitive choices and improve overall quality of service (QoS). In the 5.4 GHz band, over 225 Mega Hertz (MHz) of new spectrum was released in the United States, Europe and Brazil, and 150 MHz in Canada and Australia for unlicensed/managed devices. These devices use wideband digital modulation techniques to provide a wide array of high data rate mobile and fixed communications.
However, in many countries, wireless broadband networks and radar systems occupy the same 5 GHz frequency bands. Ensuring that interference from broadband networks does not interrupt the operations or performance of radar systems is of paramount importance. Thus a series of regulations were developed independently by a number of telecommunications regulators in various countries, including the United States, Europe, Brazil, Canada and Australia. The regulations enable coexistence of overlapping broadband networks and radar systems.
The regulatory requirements dictate that wireless broadband infrastructure and devices make use of dynamic frequency selection (DFS) radar avoidance measures to eradicate interference with government radar systems occupying the same frequency band. DFS is a feature that continually scans a frequency band and selects a channel that is not already in use. Prior to the start of a broadcast, a device equipped with DFS scans the radio surroundings for the presence of radar. If the device encounters a radar signal, it must either select another channel or enter a “sleep mode” if another channel is not available.
While DFS is highly effective at safeguarding systems and enabling coexistence between broadband networks and radar systems, effective DFS implementation methods are essential to ensure consistent and reliable outcomes for network operators. Implementation methods are complicated by the fact that countries have specified different implementation schemes, resulting in essentially two main types of DFS standards. In the United States, Canada and Australia, DFS applies only to access points and backhaul masters. For countries in the European Union and Brazil, DFS applies to access points, backhaul masters, subscriber modules and backhaul slaves.
According to some DFS specifications, when a device operating in a 5.5 GHz band identifies radar signals, it is allowed to continue operating for 260 ms to close the channel. At the end of this interval the device should search for a new channel on which to resume the operation. The check for the availability of the new channel should continue for at least one minute. Thus after a device identifies radar signals it has to warn other devices in the network that radar signals have been identified and then stop transmitting. Other devices in the network should receive the warning and should move to a new frequency where they should start a similar one minute check before transmitting any signal. If the newly selected channel is not acceptable for a particular node, that particular node must communicate that fact to the rest of the network in order to find another channel. However, because no transmission can be made before ensuring that no radar installation operates in the same frequency, it is difficult to establish coordinated communication between network nodes.
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